Low pressure discharge lamp



Dec. 18, 1962 E. LEMMERS LOW PRESSURE DISCl-iARGE LAMP Filed Feb. 9,1961 Inven tor: Eugene Le mew-s 9 His ATTOT'HGS ilnit 3,969,531 LGWPRESSURE DEQHARGE LAMP Eugene Lemrncrs, Cleveland Heights, Ohio,assignor to General Electric Company, a corporation of New Yorlr FiledFeb. 9, 1961, Ser. No. 88,228 Claims. (Cl. 313185) This inventionrelates to low pressure electric discharge lamps such as fluorescentlamps comprising a pair of thermionic electrodes sealed into oppositeends of an elongated tube containing mercury vapor and an inert gas forthe ionizable medium. The present application is a continuation-in-partof my copending application Serial No. 812,235, filed May 11, 1959, ofthe same title and assignee and now abandoned.

In the fluorescent lamp, the electric discharge through a mixture ofmercury vapor at a few microns pressure and an inert gas or mixture ofinert gases at a few millimeters pressure produces ultraviolet radiationprincipally at 2537 A. The ultraviolet radiation is converted by thephosphor coated internally on the walls of the envelope into visiblelight which is transmitted through the glass walls. The source ofultraviolet radiation resides in the excited mercury atoms and there isan optimum mercury vapor pressure, usually about six microns, at whichthe efficiency of conversion of electrical energy into ultravioletenergy is a maximum. The inert starting gas, which in the past has mostcommonly been argon at a pressure generally not less than approximately3 millimeters, is necessary principally at starting. The starting gaslowers the voltage required to initiate the arc discharge between theelectrodes and also serves to protect the electrodes from destructiveion bombardment during the starting interval.

It has been appreciated for a long time that it is possible to increasethe luminous efilciency, that is the ratio of luments output to wattsinput, in conventional fluorescent lamps such as the common 40-watt sizeby lowering the pressure of the inert starting gas. However, fluorescentlamps having argon gas filling at pressures below approximately threemillimeters yield progressively shorter life, greater end darkening andpoorer maintenance of lumen output as the pressure is reduced. Theseeffects are caused principally by more destructive positive ionbombardment of the cathodes especially at starting. In the rapid-starttype of lamp where the electrodes are heated by circulating currenttherethrough, cathode life can readily be brought back to normal byincreasing the cathode area and raising the electrode temperature to ahigher value whereby to achieve increased thermionic emission, in accordwith the teachings of Hull Patent 1,790,153. The drawback which has upto now defeated such a scheme for improving the efiiciency of thefluorescent lamp has been that the increase in cathode heating wattsrequired to maintain cathode life augments the overall losses to thepoint where the gains in efiiciency achieved by lowering the argon gaspressure are substantially completely ofiset.

The severe end darkening associated with lowered filling gas pressurecan be prevented by the use of shields about the cathodes which trap thesputtered or vaporized electrode material before it can reach thephosphor on the bulb wall. However it is now generally accepted thatshields, at least as used in the past, effectively shorten lamp life.This may be rather surprising in view of the fact that the principalpurpose of the shield has been to prevent end darkening. However I havedetermined that shields as ordinarily used to reduce end darkeningactually shortened the life of the cathode; in other words, the shieldswhile hiding the damage have accelerated it.

The principal object of the invention is to provide new and improved lowpressure electric discharge lamp such 3,6@,58l Patented Dec. 18, 1952 asfluorescent lamps having higher efficiencies than heretofore achieved.

A more specific object of the invention is to provide improvedfluorescent lamps operating at substantially the same ratings and undersubstantially the same conditions as the common sizes of fluorescentlamps which have been commercially available but having higher overallefliciency and producing greater lumen output for the same wattageinput.

Another object of the invention is to provide improved shieldedelectrode structures suitable for use in high efficiency lamps accordingto the invention.

In accordance with the invention, I have discovered that shields oranode plates about the thermionic electrodes in a low pressure dischargelamp such as a fluorescent lamp may be correlated in design to thefilamentary electrodes and particularly to the thermal inertia andoperating temperature thereof so as to achieve the desired shieldingeltect and prevention of end darkening of the lamp without anyacceleration of damage to the electrode.

Preferably lamps in accordance with the invention use an inert startinggas (including mixtures of starting gases) at a lower filling pressurethan has heretofore been commonly used and which, in the absence ofsuitable counter measures, would ordinarily result in shortened cathodelife resulting in greater end darkening and poorer maintainance of lumenoutput. However in accordance with the invention, such greater enddarkening or poorer maintenance of lumen output is substantiallyprevented through the use of shields about the filamentary electrodes towhich are correlated the electrode thermal inertia and normal operatingtemperature to prevent damage to the electrodes.

Ordinarily putting shields about a cathode as has been done in the past(in the absence of a polarized structure where the anode shield currentis caused to pass through the filament) causes damage to the cathode byseveral mechanisms. Usually the shields have been placed close to thecathode in order to trap sputtered and evaporated material moreeffectively and the general practice has been to place them so close asto produce substantial ion trapping. Such ion trapping is of courseequivalent to increase-d wall losses and alters the degree of ionizationin the sheath about the cathode. The cathode automatically makes up thisloss by adjusting to a higher cathode fall whereby to increase thedegree of ionization. A higher cathode fall results in greater ionbombardment with resulting shorter life. In addition, if the shield isconnected to the electrode inleads as it normally is, it will carry asubstantial proportion of the anode current during the anode half-cycle,the proportion depending upon the area of the shield and its positionrelative to the cathode. Current carried by the shield during the anodehalfcycle is of course current which must be subtracted from what thecathode would otherwise be carrying, with the result that the reducedcurrent flow through the filamentary cathode causes a reduction incathode temperature. Reduced cathode temperature normally entails areduction in electron emission and again the cathode must make uptherefor by readjusting to a higher cathode fall in order to increasethe electrode emission to the degree necessary to support the dischargecurrent.

In accordance with the invention, these drawbacks to the addition of ashield are circumvented by reducing the size or thermal mass of thecathode so as to achieve the optimum temperature despite the presence ofthe shields and their tendency to rob ions from the sheath and tosubtract current from the cathode. I have discovered that by using asmaller filamentary cathode designed to operate in the usual mannerwithout shields at 50% to and preferably at 50 to 62.5% of the normal 3discharge current, proper operating conditions can be remtablished atnormal current, that is at 100% current, with the use of shields. Ineffect, for the preferred range, the cathode is designed as regardsresistance and thermal mass including its disposition to lose heat byradiation and otherwise, to achieve an emission spot temperature in therange of 1050 to 1200 C. while providing 100% of the electron emissionon the cathode half cycle and only to 25% of the electron collection onthe anode half cycle. The balance of electron collection on the anodehalf cycle is made up for the most part by the shields with possibly aminor fraction by the inleads. The shields or anode plates are spacedfar enough away from the cathode, a minimum of approximately 3millimeters, that ion trapping has a negligible effect on cathode fall.

In the usual rapid start lamp, the lamp starting voltage which sustainsthe discharge current is applied at the same time as the lamp filamentvoltage which produces the current circulating through the filaments toheat them. One result of this situation is that a demand is set up forelectrons from the cathode immediately and before it has reached atemperature where a copious supply can be emitted. As a result, thedischarge which normally occurs at starting and which is frequentlyreferred to as a glow discharge occurs with a high cathode fall, forinstance as high as 100 volts and this heats the oathode and quicklyraises its temperature to the necessary electron emitting level. Howeverthe high cathode fall at starting also causes positive ion bombardmentand sputtering of the conductors including the lead-in Wires and shieldsabout the cathode. The material thus sputtered or vaporized deposits onthe cathode and reduces its electron emissivity, in effect poisoning thecathode. Existing rapid start lamps have been so designed that thedamage due to this high cathode fall or glow current at starting isinsufiicient to cause less than published or rated life. However whenshields of large area are attached to the cathode mount structure theymay take a portion of the glow current at starting by emitting electronsand thus reduce the proportion emitted by the oathode. This tends toincrease the time taken by the cathode to come up to the necessaryemitting temperature. This phenomenon of course entails more sputteredmaterial produced to poison the cathodes. Thus shields of large areawhich can operate as anodes further tend to reduce cathode life due tothe phenomena occurring at starting. In accordance with yet anotherfeature of the invention, these drawbacks are prevented by making thethermal inertia of the cathode such that even with large area shieldsconnected to the mount structure, the combined effects of cathodeheating resulting from filament heating voltage, that is, from ballastcathode voltage and glow current, will bring the filament to fullemission temperature at least as quickly as in the standard lamp.Preferably the filament is brought up and stabilized at a temperature inthe range of 850 to 1050 C. in a time interval less than 1 second afterthe application of filament heating voltage. Thus loss of life due tothe conditions at starting is substantially avoided and cathode lifeagain restored to normal.

Another feature of the invention is the use of shields or anode platesof relatively large area and consisting of a metal such as nickelpreferably, or tantalum, tungsten or molybdenum having thecharacteristic of forming a suitable base member for a barium oxidecathode. Nickel or nickel plated iron is preferred for effectiveness andlow cost. This permits a lower anode voltage drop during the anodehalf-cycle and thereby raises the elliciency.

In a preferred lamp construction embodying the invention, the foregoingfeatures of the invention are combined with a lower inert starting gasfilling pressure, for instance in the range of l to 2 millimeters, andthe starting gas consists of a mixture of argon with a minor proportionits normal current rating,

diameter of the circular section envelope from a nominal diameter of 1%.to 1% inches. In addition the preferred lamp construction makes use ofimproved phosphors which have been treated to reduce the rate ofdepreciation, such treatment being particularly desirable in view of thehigher rate of flow of ions to the wall in the instant lamp withresulting tendency towards more rapid depreciation of the phosphor.

The invention itself, together with further objects and advantagesthereof, may best be understood with reference to the followingdescription, taken in conjunction with the accompanying drawing. Thenovel features believed to be characteristic of the invention are setforth in the a pended claims.

in the drawings:

PEG. 1 is a partially cutaway perspective view of a fluorescent lamprepresentative of the invention.

FIG. 2 illustrates the mount and electrode structure of the lamp in sideelevation.

FIG. 3 illustrates the same mount and electrode struc-- ture in endview.

FIGS. 4 and 6 illustrate successive stages in making the cathodefilament. I

Referring to FIG. 1, the low pressure discharge lamp 1 embodying theinvention may correspond, in regards to its" size and generalconfiguration, to the ordinary 40-watt rapid start fluorescent lamp of48" nominal length and 1 /2 nominal diameter The lamp comprises anelongated cylindrical envelope 2 having shouldered ends to which aresecured bases 3 each provided with a pair of insulated contact terminalsor pins 4, 5. As shown at the end of the lamp where a fragment of theenvelope Wall has been broken out, electrode mount 6 comprises arelatively short stem tube 7 having its flared outer end 8 sealedperipherally into the shouldered tube end and having a press at itsinner end through which are sealed current inlead wires 11, 12. Theinward projections of the inlead wires support the filamentary cathode13 whereas the outward projections are connected to the terminal pins 4,5'. In addition the transverse extensions of the inleads support thecathode shields 14, 15. The other end of the lamp is provided with asimilar cathode and one of the stem flares is provided with an exhausttube which is sealed or tipped off in the usual fashion.

The lamp contains a quantity of mercury indicated by droplet 16exceeding in amount the quantity vaporized during operation of the lamp.The lamp contains in addition a filling of an inert starting gas, forinstance argon and neon in a proportion and at a total pressure to bemore particularly specified hereinafter. A phosphor coating indicated at17 on the inside of the envelope wall converts the resonance radiationof the discharge through the mercury vapor into visible light. The lampmay be coated externally with a water repellent substance to facilitatestarting under adverse atmospheric conditions as when the humidity ishigh.

The shields 14, 15 are eifective in reducing and substantiallypreventing the end darkening associated with lowered inert starting gaspressure. They do this by trapping: the vaporized or sputtered darkeningmaterial before it. can reach the phosphor on the bulb wall. Each shieldcomprises a main portion 18 which is disposed generally' parallel to thefilament i3 and transversely to the longi-- tudinal axis of the lamp,and an auxiliary portion 19 which extends approximately at right angles.This shield configuration serves to more or less box in the cathodewhere by to trap the darkening material more effectively. The shieldsare located about at the boundary of the cathode glow and in any case ata distance greater than approximately 3 millimeters from the emittingsurface of the cathode. For example, in a preferred construction suit;

able for a 40-watt fluorescent lamp, the distance d is 5 to 7millimeters. With this spacing, the ion trapping effect of the shieldshas negligible effect on the cathode fall.

As previously mentioned, the tendency'of the shields to reduce theoperating temperature of the cathode by subtracting from it asubstantial portion of the discharge current during the anodehalf-cycle, is ofiset by using a smaller cathode which heats up morequickly and which will be maintained at the required electron emittingtemperature, for instance in the range of approximately 900 to 1000 C.,under the conditions encountered. I have found that in general a cathodewhich is designed to operate in the usual manner, without shields, at acurrent which is 50 to 62.5% of the normal lamp current, will operate inthe desired fashion at normal current with shields.

As an example of a cathode in accordance with the invention suitable fora 40-watt fluorescent lamp and utilizing an overwind in accordance withthe teachings of US. Fatent 2,306,925, Aicher, the followingconstruction may be used. Referring to KG. 4, the first coiling providesthe overwind convolutions consisting of 0.4 mil tungsten wire 21 wrappedat 290 turns per inch around a composite mandrel formed by a 1.9 miltungsten Wire 22 and a 1.5 mil molybdenum wire 23 laid alongside eachother to provide a composite wire 24-. In the second coiling illustratedin PEG. 5, the product 24 of the first coiling is wrapped at 13% turnsper inch around a mandrel consisting of a 3.5 mil molybdenum wire 25 toprovide a composite wire 2-5. In the illustration of FIG. 5, molybdenummandrel wire 23 has been omitted for the sake of clarity; also in FIG.6, both molybdenum wires 23 and 25 are omitted for the same reason. Inthird coiling illustrated in FIG. 6, the product as of the secondcoiling is Wrapped around a mandrel ccnsistirn of a 13.5 mil steel pin,the cathode coil being thereafter removed from it by slipping it off.The cathode coil or filament 13 may consist of approximately 18 turns ofthe final coiling with straight extensions or legs which are clampedbetween the folded inner ends of the inlead wires 11, 12. Previous toclamping between the inlead wires, the cathode convolutions are set bysuitable heat treatment and the molybdenum mandrel wires 23 and 25 areremoved by dissolving in acid. Thus the filament 13 is a hollow orskeletal structure consisting of the triple-coiled overwind wire 21fitting loosely around the turns of the double-coiled mandrel wire 22.This particular filament is merely an example of a cathode meeting therequirements of the invention in regards to correlation with shielddesign so that the cathode carries 50 to 62.5% of the total dischargecurrent. Other cathode structures may readily be proportioned to the sare criteria. including if desired simpler cathode structures not using anoverwind or triple coiling and using for instance only double coiling.

The shield structure comprises a pair of thin nickel strips 14, 15, eachhaving a width W of approximately 8 mm. and a total length L ofapproximately 21 mm. The short leg 1? of each stri welded to the inleadis approximately 6 mm. long. and the long leg 18 approximately mm. long.The long legs are preferably positioned parallel to the filament asillustrated in FIG. 3. The distance D between strips is approximately 15mm. and the overt'll shield structure surrounds the filament in a planetransverse to the longitudinal axis of the lamp and is generallybox-like around the filament. The distance d from the emitting surfaceof the filament to the shield strip is ap proximately 6 millimeters, adistance substantially greater than the minimum of 3 millimetersrequired to reduce ion trapping to the point where it has negligibleefiect on cathode fall. The total inside surface of the shield members14, 15 surrounding the cathode is approximately /2 in. or 3.3 cm.However this is not critical and may be varied within the principlesstated, and in particular may be inc eased substantially. With the givendimensions, the cathode or filament carries from 12 to 18% of the 6current on the anode half-cycle so that the cathode supports, on a fullcycle average, 56 to 59% of the total discharge current.

The usual procedures are followed in making a lamp using the cathodes ofthe present invention. Previous to mounting the lamp envelope, thecathode mounts are processed to coat the filamentary cathodes withactivating material such as barium, strontium, and cal ium carbonates ormixtures thereof. The mounts are then sealed to the ends of the glassenvelope and the electrodes are activated during lamp manufacture bypassing a heating current through them to reduce the carbonates tooxides.

The shields are preferably made of a metal which forms a good basemember for activation by barium or barium oxide. Suitable metals aretungsten, molybdenum and nickel; nickel-plated iron is satisfactory andis preferred because it is cheapest. The shields of nickel-plated ironbecome sotospeak activated during operation, and this reduces the anodefall and permits an increase of 3 to 5% in lamp efiiciency. Theactivation of the shields comes about as a result of the depositionthereon of evaporated emission material, principally barium, sputteredor vaporiZed from the cathodes. A surface which is a good electronemitter is also a good electron collector and as a result when theshields operate as anodes during the anode half-cycle, the anode voltagedrop and of course the lamp voltage is decreased by several volts. Ifdesired, the shields may be made of screening or else of perforatedstrip in order to reduce trapping of 2537 A. radiation originating fromthe cathode glow. The shield plates need not necessarily be L-shaped asdescribed and illustrated; another convenient form consists in C-shapedshield seg ments whereby the shield encircles the filament in a circular or ring-like configuration.

With the shields placed in the preferred position about the cathodes,namely the outer fringes of the expanded cathode glow, and with theinert filling gas pressure lowered into the range of l to 2 millimeters,a 48" long 1 /2" diameter lamp is found to consume only about 37 wattsin order to produce the same lumens output as the standard 40-watt lampwhile at the same time realizing an efficiency approximately 8% higher.

In the foregoing lamp which requires an input of 37 watts only toproduce the same lumens output as the prior art 40-watt lamps, thewattage input may be restored to the 40-watt level and the total lumenoutput raised proportionately by decreasing the bulb diameter or bychanging the inert starting gas composition. The latter may be done inaccordance with the teachings of copending application Serial No.812,236, filed May 11, 1959, of Kurt Schmidt, entitled Fluorescent LampGas Filling, and assigned to the same assignee as the present invention.In accordance therewith, at relatively low loadings in the range of 5 to15 watts per foot, an increase in efiiciency may be achieved byreplacing up to approximately 50% of the usual argon filling gas by neonin the pressure range from 1.5 to 3 millimeters. At a pressure ofapproximately 2 millimeters, the preferred proportions of neon is in therange from 25 to 40%, the latter being preferred where the emphasis ison maximum output, and the former being preferred where the emphasis ison maximum life. This allows an improvement in efficiency ofapproximately 2% over the maximum efficiency which may be achieved withpure argon at the same total pressure. For the instant 40-watt lamp, thepreferred filling gas mixture consists of 65% argon and 35% neon at atotal pressure of approximately 2 millimeters of mercury.

It is known that reducing the filling pressure of the inert starting gasor using a starting gas of low atomic weight such as neon in lieu ofpart of the argon, entails a greater rate of depreciation of thephosphor coating. The foregoing measure increases the rate of flow ofions to the wall which appears to be responsible for a reactionoccurring between the constituents of the bulb glass and the phosphor.Among the end products of this reaction are a phosphor surface which ismercurophilic, that is one to which mercury is more attracted andbecomes more readily attached. The attachment of mercury to the phosphorsurface causes a reduction in lumen output because the mercury layer isopaque to both the exciting ultraviolet radiation and to the emittedvisible light. To reduce this deleterious effect as much as possible inthe instant lamp, the phosphor may be treated to remove the surfacemetallic ions such as antimony, manganese, etc. Also the envelope wallmay be treated to remove alkali materials therefrom whereby to make it abetter insulator and also to leave less material for reaction with thephosphor surface or to serve as attachment points for mercury. Anotherprocedure which may be used is to apply a potential to the bulb wallwith such polarity that all the electro-positives such as sodium aremoved to the outer surface.

Comparative tests of the foregoing improved 40-watt lamp in accordancewith the invention and of regular production 40-watt rapid startfluorescent lamps produced the following results. The cathode (filament)temperature of the regular lamp at nominal cathode heating voltage (3.65volts) stabilized at approximately 930 C. in 1.6 seconds after initalapplication of voltage, and the temperature of the emission spot on thecathode with normal discharge current (430 milliamperes) wasapproximately 1130 C. In the improved lamp using the rapid heatinglament and the shields and a filling of 65% argon and- 35% neon at atotal pressure of 2 millimeters of mercury, the cathode (filament)temperature stabilized at approximately 985 C. in 0.6 second afterinitial application of voltage, and the temperature of the emission spotwith normal discharge current was approximately 1125" C. The foregoingtemperatures were measured with an optical pyrorneter focused on theoxide coated surface of the electrode and include no correction fordeparture from black body radiation. Thus the improved lamp stabilizesas to cathode temperature in one half to one third the time and hassubstantially the same emission spot temperature despite the large areashields which operate as anodes. By comparison, When the shields aremerely added about the cathodes of the regular lamp, the emission spottemperature drops from 1130 C. to 970 C., a value too low forsatisfactory life and maintenance.

The improved 40-watt lamp in accordance with the invention realizes animprovement in efficiency of 8% over prior lamps, and for the first timemakes possible an output in excess of the 3000 lumens level for an inputof 40 watts. By using in this lamp more eflicient phosphors ofcontrolled particle size for maximum brightness, an output of 3100lumens at an efliciency of 77.5 lumens per watt is achieved. Such highefficiency has never belore now been rcaiized in a 40-watt lamp.

While a certain specific embodiment of the invention has beenillustrated and described in detail, various modifications will readilyoccur to those skilled in the art inasmuch as the underlying principlesmay readily be applied to different sizes of lamps. The elongateddischarge channel may be otherwise than straight and tubular; forinstance a channel of re-entrant cross-section may be used, or a curvedchannel as in circline lamps, or a sinuous channel as in labyrinthinepanel lamps. The appended claims are therefore intended to cover anysuch modifications coming Within the true spirit and scrpe of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to achieve an electron emission spot temperature in therange of 1050 to 1200" C. with emission of d.'s:harge current on thecathode half cycle and 0 to 25% eclectic-n on the anode half cycle, andconductive shields fastened to said inleads encompassing the cathodeglow region of said electrode and collecting substantially the balanceof discharge current.

2. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to achieve an electron emission spot temperature in therange of 1050 to 1200 C. with 100% emission on the cathode half cycleand 0 to 25% collection on the anode half cycle, and conductive shieldsfastened to said inleads generally surrounding said filament andencompassing its cathode glow region, said conductive shields consistingof metal subject to activation by deposition thereon of activatingmaterial from said filament and being substantial in surface areawhereby to collect substantially the balance of discharge current on theanode half cycle with a low anode voltage drop.

3. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to stabilize at a temperature in the range of 850 to 1050C. in a time interval less than 1 second after the application offilament heating voltage and to achieve in operation an electronemission spot temperature in the range of 1050 to 1200 C. with 100%emission of discharge current on the cathode half cycle and 0 to 25%collection on the anode half cycle, and conductive shields fastened tosaid inleads encompassing the cathode glow region of said electrode andcollecting substantially the balance of discharge current.

4. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to stabilize at a temperature in the range of 850 to 1050C. in a time interval less than 1 second after the application offilament heating voltage and to achieve in operation an electronemission spot temperature in the range of 1050 to 1200 C. with 100%emission on the cathode half cycle and 0 to 25% collection on the anodehalf cycle, and conductive shields fastened to said inleads generallysurrounding said filament and encompassing its cathode glow region, saidconductive shields consisting of metal subject to activation bydeposition thereon of activating material from said filament andcollecting substantially the balance of discharge current on the anodehalf cycle.

5. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to stabilize at a temperature in the range of 850 to 1050C. in a time interval less than 1 second after the application offilament heating voltage and to achieve in operation an electronemission spot temperature in the range of 1050 to 1200 C. with 100%emission on the cathode half cycle and to 25% collection on the anodehalf cycle, and a pair of conductive shields fastened to said inleadsgenerally surrounding said filament and located at a distance from theemission spot surface of said filament not less than approximately 3millimeters and disposed at about the limits of the cathode glow region,said conductive shields being substantial in surface area and consistingof metal subject to activation by deposition thereon of activatingmaterial from said filament whereby to collect substantially the balanceof discharge current on the anode half cycle with a low anode voltagedrop.

6. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas, each electrodestructure comprising a multiple coiled tungsten filament coated withalkaline earth oxide activating material and supported at opposite endsby inlead wires sealed through said envelope, said filament beingproportioned in resistance and thermal mass to stabilize at atemperature in the range of 850 to 1050 C. in a time interval less than1 second after the application of filament heating voltage and toachieve under operating conditions an emission spot temperature in therange of 1050 to 1200 C. with 100% emission on the cathode half cycleand 0 to 25 collection on the anode half cycle, and a pair of conductiveshields fastened to said inleads generally surrounding said filament ina plane transverse to the longitudinal axis of said channel thereat andlocated at a distance from the center of said filament not less thanapproximately 3 millimeters and disposed at about the limits of thecathode glow region, said conductive shields being substantial insurface area and consisting of metal subject to activation by depositionthereon of activating material from said filament whereby to collectsubstantially the balance of discharge current on the anode half cyclewith a low anode voltage drop.

7. A low pressure electric discharge lamp comprising an elongatedvitreous envelope having a pair of electrode structures at opposite endsand containing gas consisting of an ionizable medium comprising mercuryvapor and an inert starting mixture of argon and not over 50% neon at atotal pressure less than 3 millimeters of mercury, each electrodestructure comprising a coiled tungsten filament coated with activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to achieve an electron emission spot temperature in therange of 1050 to 1200 C. with 100% emission of discharge current on thecathode half cycle and 0 to 25% collection on the anode half cycle, andconductive shields fastened to said inleads encompassing the cathodeglow region of said electrode and collecting substantially the balanceof discharge current.

8. A low pressure electric discharge lamp comprising an elongatedvitreous envelope having a pair of electrode structures at opposite endsand containing and ionizable medium comprising mercury vapor and aninert starting easm gas consisting of a mixture of argon and not over50% neon at a total pressure less than 3 millimeters of mercury, eachelectrode structure comprising a coiled tungsten filament coated withactivating material and supported at opposite ends by inlead wiressealed through said envelope, said filament being proportioned inresistance and thermal mass to stabilize at a temperature in the rangeof 850 to 1050 C. in a time interval less than 1 second after theapplication of filament heating voltage and to achieve an electronemission spot temperature in the range of 1050 to 1200 C. with emissionof discharge current on the cathode half cycle and 0 to 25% collectionon the anode half cycle, and conductive shields fastened to said inleadsencompassing the cathode glow region of said electrode and collectingsubstantially the balance of discharge current.

9. A low pressure electric discharge lamp comprising an elongatedvitreous envelope having a pair of electrode structures at opposite endsand containing an ionizable medium comprising mercury vapor and an inertstarting gas consisting of a mixture of argon with appoximately 25 to40% neon at a total pressure of approximately 2 millimeters of mercury,each electrode structure comprising a coiled tungsten filament coatedWith activating material and supported at opposite ends by inlead wiressealed through said envelope, said filament being proportioned inresistance and thermal mass to stabilize at a temperature in the rangeof 850 to 1050 C. in a time interval less than 1 second after theapplication of filament heating voltage and to achieve an electronemission spot temperature in the range of 1050 to 1200 C. with 100%emission of discharge current on the cathode half cycle and 0 to 25%collection on the anode half cycle, and conductive shields fastened tosaid inleads encompassing the cathode glow region of said electrode andcollecting substantially the balance of discharge current.

10. A low pressure fluorescent lamp comprising an elongated vitreousenvelope coated internally with a phosphor and having a pair ofelectrode structures at opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas consisting of amixture of argon and 25 to 40% neon at a total pressure of approximately2 millimeters of mercury, each electrode structure comprising a multiplecoiled tungsten filament coated with alkaline earth oxide activatingmaterial and supported at opposite ends by inlead wires sealed throughsaid envelope, said filament being proportioned in resistance andthermal mass to stabilize at a temperature in the range of 850 to 1050C. in a time interval less than 1 second after the applicationoffilament heating voltage and to achieve in operation an electronemission spot temperature in the range of 1050 to 1200 C. with 100%emission of discharge current on the cathode half cycle and 0 to 25%collection on the anode half cycle, and a pair of conductive shieldsfastened to said inlead generally surrounding said filament in a planetransverse to the longitudinal axis of said envelope and located at adistance from the center of said filament not less than approximately 3millimeters and disposed at about the limits of the cathode glow region,said conductive shields being substantial in surface area and having anickel surface subject to activation by deposition thereon of activatingmaterial from said filament whereby to collect substantially the balanceof discharge current on the anode half cycle with a low anode voltagedrop.

No references cited

